In the petrochemical industry, a gas flow meter (typically referred to as a “meter run”) may be installed in a section of pipe to monitor the amount of natural gas flowing through the pipe. For gas flow meters that monitor flow volume by sensing gas pressure upstream and downstream of a flow restriction device (such as an orifice plate), the differential pressure of gas flow through the flow meter will affect the accuracy of the readings that are obtained and it has been found that the meter is most accurate at around 50%, losing accuracy when significantly above or below that level. As such, it is advantageous to be able to change the aperture size of the flow restriction device, in order to vary the gas flow through the restriction device and thereby select the desired pressure differential, without interrupting the flow of gas through the flow meter and the pipeline for a significant period of time.